Ultragenyx Receives FDA Approval for Sanfilippo Syndrome Gene Therapy

Sanfilippo syndrome is a rare inherited metabolic disorder that causes severe developmental regression in children, who often do not survive past their teenage years. In September 2026, the U.S. Food and Drug Administration approved Fayuvi, a gene therapy for the condition, marking the first FDA clearance for this specific treatment.

The approval validates a research pathway initiated by Haiyan Fu at Nationwide Children's Hospital in Columbus, Ohio. Fu began developing the therapy in the late 1990s using a modified virus to deliver a functional copy of the defective gene into cells. The project faced financial hurdles, including a potential halt in 2009, before eight patient foundations, including the Cure Sanfilippo Foundation, provided the necessary funding to continue the work.

The Cure Sanfilippo Foundation was established by Glenn and Cara O'Neill after their daughter, Eliza, received a diagnosis at age three. To fund the first human clinical trial, which began in May 2016, the family launched a crowdfunding campaign. The campaign raised $2 million in a few weeks in 2014, setting a record for the platform at the time. Eliza became the first patient to receive the experimental infusion.

In 2022, biotechnology company Ultragenyx acquired the rights to the therapy and submitted the data for regulatory review. Emil Kakkis, CEO of Ultragenyx, stated that early intervention yields better outcomes, noting that children treated at six months of age showed strong results. The company set the price for the one-time infusion at nearly $4 million, a figure that ties it for the second most expensive gene therapy on the market, according to drug price research firm 46brooklyn.

Eliza O'Neill, now nearly 17, received the treatment at age six. Although she had already lost the ability to speak by the time of her infusion, she now communicates using picture cards and devices and attends school. Fu, who received a call from Cara O'Neill immediately after the approval announcement, expressed mixed emotions, stating that the timeline for bringing the therapy to patients was longer than desired.

Ultragenyx has indicated it will ensure patient access regardless of cost. The company plans to continue studying the therapy while making it available to eligible patients. The approval concludes a multi-year effort that began with family-funded research and ended with a major regulatory decision.